Government proposals to criminalise squatting may also outlaw occupation-style protests and sit-ins, student leaders, trades unionists and lawyers have said.

Plans to make trespass a criminal offence appear in a recently published justice ministry consultation paper that seeks to make it easier to evict squatters from unoccupied commercial and residential property. Currently, trespass is a civil offence and property owners must take squatters through the civil courts.

Lawyers fear that outlawing trespass, the ministry's strongest proposal, will also have the effect of banning peaceful sit-ins, a protest tactic that is favoured by students and workers.

National Union of Students national executive member Michael Chessum said the NUS would mount a campaign to make it "politically impossible" for the government to introduce such a law.

"It will be a fundamental affront to the right of students to protest if occupation was criminalised," Chessum said.

"I have no doubt that if the government tries to criminalise occupations … we would mount a campaign to make it politically impossible to stop student occupiers occupying." Last autumn thousands of students occupied dozens of campuses in an attempt to stop a tripling of tuition fees to £9,000.

Chessum added that criminalising trespass could change the environment on campus and lead to "war" between management and students involving police and courts.

"The idea that peacefully occupying your own campus might be taken away from you is something that we simply won't take," he said.

He added that many students also squat and the proposals would also be resisted on this basis.

"Students and young people in particular are thrown out into a world in which there are vanishingly few jobs and housing is an absolutely massive crisis [for them] … and will defend the right to squat" he said.

Teresa Mackay, a regional organiser for the Unite union, said anti-squatting legislation had become a big issue for unions and they were planning to submit their concerns to the government's consultation process, which closes on 5 October.

"I have spoken to other trade unionists about this and they do feel that almost that the attack on squatters is a bit of a smokescreen when you think that of the battles that are going to possibly take place over the next few years," she said. In a statement, the Ministry of Justice (MoJ) denied that its intention was to criminalise occupations and said the consultation was aimed specifically at squatting. On page nine of the consultation it suggests that any new law might want to exempt occupation protests from criminalisation.

But Giles Peaker, a housing solicitor, said it would be difficult for the legal system to distinguish between squatting and politically-based occupations.

"The consultation suggests that certain kinds of occupation might possibly be excluded, including things like students occupying university property, it doesn't say that they will be, and the suggestion appears to be you would have Ministry of Justice licensed protests – permitted and not permitted.

"Is it necessarily for the MoJ to be the ones proscribing what constitutes a valid form of protest and what doesn't? … The potential for legal grey areas is huge."

Cat Brogan who was evicted by St John Hackney Joint Charities trust from a squat in Well Street, Hackney last month, said that if squatting was criminalised it would be "unaffordable and unenforceable" and would put the burden on tax payers.

"[Criminalising squatting] means that you are using taxpayers money to pay for costly evictions for rich landlords and rich property owners who have vast swathes of property that they leave empty for land banking," she said.

Responding to fears an MoJ spokesperson said: "These proposals are not targeting legitimate forms of protesting but those people who enter and occupy homes or business properties without permission.

"No decisions on our squatting proposals have been made and we await with interest the responses to the consultation which is due to be published this autumn.

"Our consultation will tell us more about the types of people who squat and the extent of squatting. We particularly welcome the views of any organisations that are likely to be affected by our proposals."